Method of producing dispersion-hardened metal alloys



United States Patent 3,203,781 METHOD OF PRODUCING DISPERSION HARDENED METAL ALLOYS Karl Bungardt, Krefeld, Germany, assignor to Deutsche Edelstahlwerke Aktiengesellschaft, Krefeld, Germany No Drawing. Filed Sept. 26, 1963, Ser. No. 311,621 Claims priority, application Germany, Oct. 2, 1962,

12 Claims. (Cl. 75-5 It is a well known fact that the high temperature strength of metals and alloys can be substantially improved by incorporating finely divided hard particles therein which are not soluble in the metal or in the alloy. Suitable hard substances for this purpose include high melting oxides, such as aluminium or cerium oxide, high melting nitrides, such as titanium or aluminium nitride, and others. The particle size and distribution of the embedded materials decisively affect the resultant strength. Moreover, since the particles of hard substances are insoluble in the base metal, there is no coagulation at higher temperatures. Materials of such a kind are referred to in the literature as dispersion hardened alloys.

Diverse methods have been proposed for producing dispersion hardened alloys. One known method consists in adding the hard substances, such as oxides, to the molten metal in powder form. However, when this is done it is observed that the particles of hard material tend to coagulate, a tendency which has an adverse effect upon strength. Better results are achieved by powder metallurgical methods, for instance by mixing powders of the metal and hard substance after these have been ground, followed by pressing and sintering of the mixture.

Another known'method is that known as internal oxi-,

dation of metal alloys. For performing this method at least one alloying component should have as high as possible an affinity for oxygen, such as aluminium, whereas the base element of the alloy should have a low affinity for oxygen. A suitable base element would be nickel. The alloy is subjected to an oxidation treatment at elevated temperature, causing the aluminium to be converted into its oxide, Whereas the nickel remains 'substantially unoxidised. In order to avoid extremely long oxidation times the alloy is powdered before submitted to the oxidation treatment so that the particle cross sections are small. The powder which has thus been treated and reheated in a hydrogen atmosphere to remove any nickel oxide that may have been formed, is then further processed by pressing, sintering and possibly by further compaction, for instance by extruding the same.

The degree of fineness which is achieved and the resultant good distribution of the oxides dispersed in the alloy by internal oxidation are responsible for imparting to alloys produced by this process greater improvement in strength over that of the base metal than has ever been achieved in the past. Nevertheless, the method is expensive and tiresome to perform.

More recently a different process has been proposed which gives results that are as good as those achieved by internal oxidation. In this process the metal powder and the powdered hard substance are ground, mixed and then processed by pressing, sintering and after-compaction. However, grinding aids are added to the powders, such as aluminium nitrate or cerium nitrate, so that the powder can be ground down to a high degree of fineness.

It may, therefore, be expected that a further reduction and an even better distribution of the hard substance particles will permit the strength of the resultant alloy to be even further improved. The problem of introducing hard substances into alloys and of distributing them I 3,203,781 PatentedAug. 31, 1965 therein as finely and evenly as possible is therefore one that calls for further attention.

Surprisingly it has now been found that by melting dispersion hardened alloys which have been produced by one of the above described methods their strength can 41 fact be further increased. A method which produces favourable results is that of melting in an electric arc vacuum furnace, preferably into a cooled permanent mould. This operation may be carried out either in a vacuum or in a protective gas atmosphere. Excellent results are also achieved by melting in an electron beam furnace.

The initial compacts are produced in conventional manner, that is to say, for instance, powder metallurgically by grinding, pressing and possibly sintering. The usual degree of dispersion is thus achieved and the hard substances introduced into the alloy have the usual degree of fineness. Melting in an electric arc vacuum furnace has the effect, on the one hand, that the metal alloy is further homogenised throughout the cross section and, on the other hand, that the high energy applied to the surface of the bath agitates and probably further divides up the hard substances which are dispersed in the alloy. Finally, the motion of the bath during the melting process causes a further'size reduction and equalisation of particle distribution. Since because of the cooling effect of the moulds the depth of the bath in such furnaces is small, the period during which the alloy is liquid is not long enough to permit the very finely divided particles to reco-agulate. The result is a more even distribution of the dispersed particles and hence improved mechanical properties of the alloy.

It has been observed that alloys produced by the method according to the invention can be directly further treated by forging, rolling or extrusion. Appropriate hard substances for the proposed method are oxides, nitrides, carbides or the like in quantities between 0.5% and 15%, preferably between 2% and 10%. The effect of the hard substance additions is all the more pronounced the lower their solubility in the base metal.

The following two embodiments of the invention are given by way of example:

Example 1 According to one embodiment of the invention a powdered nickel was mixed with about 5% aluminium oxide and pressings were produced from the mixture. These pressings were sintered for about 4 hours in an argon atmosphere at 1150 C. and then cooled to room temperature. The bodies thus obtained were melted in an electric arc vacuum furnace equipped with a watercooled permanent copper mould. Upon completion of the melting process specimens were forged from the contents of the mould. These forgings contained 4.63% of aluminium oxide.

Example 2 A finely powdered alloy consisting of nickel and 1.5% aluminium was compacted into pressings. Before being sintered the pressings were submitted to a thermal treatment by keeping them at 800 C. in air for four hours. At the end of these 4 hours the bodies were cooled to room temperature and then reheated in a hydrogen atmosphere and kept at 450 C. for 6 hours. This latter operation caused the previously formed nickle oxide to be reduced, whereas the aluminium oxide was not changed.

Finally the bodies which had thus been treated were sintered in an argon atmosphere at 1150 C. and then melted down in an electric arc vacuum furnace. A watercooled copper mould was again used.

The melted contents of the mould were forged and rolled down into sheets. The aluminium oxide content of the sheets proved to be 2.5% A1 It has been found that the new method produces a further size reduction of the hard metal particles which are dispersed into the base metal, even if these are originally already extremely fine and have been produced say be the method of internal oxidation.

What I claim is:

1. A method of preparing a dispersion-hardened metal alloy which comprises preparing a first dispersion-hardened metal alloy including a metal matrix and hard nonmetallic particles distributed throughout said matrix, said hard particles being insoluble in said matrix; melting said first metal alloy; and then solidifying said first alloy melt, said melting and solidification of said first alloy reducing the size and improving the distribution of the hard particles in the matrix to form a second alloy of increased strength.

2. The method according to claim 1 wherein the said first alloy is melted in the form of a self-consuming electrode in an electric arc furnace and solidified in a cooled permanent mould.

3. The method according to claim 2 wherein said melting is carried out in a vacuum.

4. The method according to claim 2 wherein said melting is carried out in a protective gas atmosphere.

5. The method according to claim 1 wherein said first alloy is a pre-prepared dispersion-hardened alloy that is melted in the form of a sintered compact produced by powder metallurgy.

6. The method according to claim 1 wherein said first alloy is a pre-prepared coherent alloy that is melted in an electron beam furnace.

7. The method according to claim 1 which further comprises the step of subjecting the solidified second alloy to a mechanical forming process.

8. The method according to claim 1 wherein the said hard particles are comprised of at least one hard oxide material which is incorporated in the metal matrix alloy by internal oxidation.

9-? A method of preparinga dispersion-hardened metal alloy which comprises preparing a first dispersion-hardened metal alloy including a metal matrix and hard particles distributed in said matrix, said hard particles being insoluble in said matrix and comprising at least one material selected from the group consisting of oxides, nitrides and carbides; melting said first metal alloy; and then solidifying said first alloy melt, said melting and solidification of said first alloy reducing the size and improving the distribution of the hard particles in the matrix to form a second alloy of increased strength.

10. The method according to claim 9 wherein the said hard particles are incorporated in the metal matrix in a quantity of between 2 percent and 10 percent.

11. The method according to claim 9 wherein the said hard particles are incorporated in the metal matrix in a quantity of between 0.5 percent and 15 percent.

12. The method according to claim 1 wherein the said first alloy is melted in an electric arc.

References Cited by the Examiner UNITED STATES PATENTS 2,782,114 2/57 Preston -10 2,819,158 1/58 Johnston 7510 2,866,700 12/58 Bohnet et al 75--10 2,890,109 6/59 Cooper 75-10 FOREIGN PATENTS 1,255,349 10/59 France.

OTHER REFERENCES Jackson, article in Battelle Technical Review, October 1958, vol. 7, No. 10, pp. 8-12.

Van Thyne et al., article in Iron Age, Aug. 6, 1953, pp. 146-148.

DAVID L. RECK, Primary Examiner. 

1. A METHOD OF PREPARING A DISPERSION-HARDENED METAL ALLOY WHICH COMPRISES PREPARING A FIRST DISPERSION-HARDENED METAL ALLOY INCLUDING A METAL MATRIX AND HARD NONMETALLIC PARTICLES DISTRIBUTED THROUGHOUT SAID MATRIX, SAID HARD PARTICLES BEING INSOLUBLE IN SAID MATRIX; MELTING SAID FIRST METAL ALLOY; AND THEN SOLIDIFYING SAID FIRST ALLOY MELT, SAID MELTING AND SOLIDIFICATION OF SAID FIRST ALLOY REDUCING THE SIZE AND IMPROVING THE DISTRIBUTION OF THE HARD PARTICLES IN THE MATRIX TO FORM A SECOND ALLOY OF INCREASED STRENGTH. 